Solid electrolyte material and battery using same

ABSTRACT

The solid electrolyte material consists essentially of Li, Ti, M, and F. Here, M is at least one selected from the group consisting of Mg and Ca.

BACKGROUND 1. Technical Field

The present disclosure relates to a solid electrolyte material and abattery using it.

2. Description of the Related Art

Japanese Unexamined Patent Application Publication No. 2011-129312discloses an all solid battery using a sulfide solid electrolyte.Japanese Unexamined Patent Application Publication No. 2008-277170discloses LiBF₄ as a fluoride solid electrolyte material.

SUMMARY

One non-limiting and exemplary embodiment provides a solid electrolytematerial having a high lithium ion conductivity.

In one general aspect, the techniques disclosed here feature a solidelectrolyte material consisting essentially of Li, Ti, M, and F, whereinM is at least one selected from the group consisting of Mg and Ca.

The present disclosure provides a solid electrolyte material having ahigh lithium ion conductivity.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a battery 1000 according to a secondembodiment;

FIG. 2 is a cross-sectional view of a battery 2000 according to thesecond embodiment;

FIG. 3 is a schematic view of a compression molding dies 300 used forevaluation of the ion conductivity of a solid electrolyte material;

FIG. 4 is a graph showing a Cole-Cole plots obtained by impedancemeasurement of the solid electrolyte material of Example 1; and

FIG. 5 is a graph showing the initial discharge characteristics ofbatteries of Example 1 and Comparative Example 1.

DETAILED DESCRIPTIONS

Embodiments of the present disclosure will now be described withreference to the drawings.

First Embodiment

The solid electrolyte material according to a first embodiment includesLi, Ti, M, and F. M is at least one selected from the group consistingof Mg and Ca. The solid electrolyte material according to the firstembodiment has a high lithium ion conductivity. Here, a high lithium ionconductivity is, for example, 1×10⁻⁸ S/cm or more. That is, the solidelectrolyte material according to the first embodiment can have an ionconductivity of, for example, 1×10⁻⁸ S/cm or more.

The solid electrolyte material according to the first embodiment can beused for obtaining a battery having excellent charge and dischargecharacteristics. An example of the battery is an all solid battery. Theall solid battery may be a primary battery or may be a secondarybattery.

The solid electrolyte material according to the first embodimentdesirably does not contain sulfur. A solid electrolyte material notcontaining sulfur does not generate hydrogen sulfide, even if it isexposed to the atmosphere, and is therefore excellent in safety. Thesulfide solid electrolyte disclosed in Japanese Unexamined PatentApplication Publication No. 2011-129312 may generate hydrogen sulfidewhen exposed to the atmosphere.

The solid electrolyte material according to the first embodimentcontains F and can therefore have a high resistance to oxidation. Thisis because F has a high redox potential. At the same time, since F has ahigh electronegativity, the bond with Li is relatively strong. As aresult, generally, the lithium ion conductivity of the solid electrolytematerial containing Li and F is low. For example, LiBF₄ disclosed inJapanese Unexamined Patent Application Publication No. 2008-277170 has alow ion conductivity of 6.67×10⁻⁹ S/cm. Incidentally, LiBF₄ is the solidelectrolyte material used in Comparative Example 1 described later. Incontrast, the solid electrolyte material according to the firstembodiment further includes Ti and M, in addition to Li and F, andthereby can have a high ion conductivity of, for example, 1×10⁻⁸ S/cm ormore.

In order to enhance the ion conductivity of the solid electrolytematerial, the solid electrolyte material according to the firstembodiment may include an anion other than F. Examples of the anion areCl, Br, I, O, S, and Se.

The solid electrolyte material according to the first embodiment mayconsist essentially of Li, Ti, M, and F. Here, the phrase “the solidelectrolyte material according to the first embodiment consistsessentially of Li, Ti, M, and F” means that the molar proportion (i.e.,molar fraction) of the sum of the amounts of Li, Ti, M, and F to the sumof the amounts of all elements constituting the solid electrolytematerial according to the first embodiment is 90% or more. As anexample, the molar proportion may be 95% or more. The solid electrolytematerial according to the first embodiment may consist of Li, Ti, M, andF only.

The solid electrolyte material according to the first embodiment maycontain elements that are unavoidably mixed. Examples of the elementsare hydrogen, oxygen, and nitrogen. These elements can be present in theraw material powders of the solid electrolyte material or in theatmosphere for manufacturing or storing the solid electrolyte material.

In order to further enhance the ion conductivity of the solidelectrolyte material, in the solid electrolyte material according to thefirst embodiment, the ratio of the amount of Li to the sum of theamounts of Ti and M may be 0.5 or more and 4.5 or less.

In order to enhance the ion conductivity of the solid electrolytematerial, M may be Mg.

The solid electrolyte material according to the first embodiment may berepresented by a composition formula (1):Li_(6-(4-2x)b)(Ti_(1-x)M_(x))_(b)F₆, where mathematical expressions:0<x<1 and 0<b≤3 are satisfied. The solid electrolyte material havingsuch a composition has a high ion conductivity.

In order to enhance the ion conductivity of the solid electrolytematerial, in the formula (1), a mathematical expression: 0.05≤x≤0.9 maybe satisfied.

When M is Mg, in order to enhance the ion conductivity of the solidelectrolyte material, in the formula (1), a mathematical expression:0.05≤x≤0.6 may be satisfied.

When M is Ca, in order to enhance the ion conductivity of the solidelectrolyte material, in the formula (1), a mathematical expression: x=0may be satisfied.

The upper limit and lower limit of the range of x in the formula (1) canbe defined by an arbitrary combination selected from the numericalvalues of 0.05, 0.1, 0.2, 0.4, 0.5, 0.6, and 0.8.

In order to enhance the ion conductivity of the solid electrolytematerial, in the formula (1), a mathematical expression: 0.80≤b≤1.71 maybe satisfied.

The upper limit and lower limit of the range of b in the formula (1) canbe defined by an arbitrary combination selected from the numericalvalues of 0.8, 0.86, 0.9, 1.0, 1.1, 1.3, 1.5, and 1.71.

The solid electrolyte material according to the first embodiment may becrystalline or amorphous.

The shape of the solid electrolyte material according to the firstembodiment is not limited. Examples of the shape are needle, spherical,and oval spherical shapes. The solid electrolyte material according tothe first embodiment may be a particle. The solid electrolyte materialaccording to the first embodiment may be formed so as to give a pelletor planar shape.

For example, when the solid electrolyte material according to the firstembodiment has a particulate (e.g., spherical) shape, the solidelectrolyte material may have a median diameter of 0.1 μm or more and100 μm or less. The median diameter means the particle diameter at whichthe accumulated volume is equal to 50% in a volume-based particle sizedistribution. The volume-based particle size distribution is measuredwith, for example, a laser diffraction measurement apparatus or an imageanalyzer.

The solid electrolyte material according to the first embodiment mayhave a median diameter of 0.5 μm or more and 10 μm or less.Consequently, the solid electrolyte material has a higher conductivity.Furthermore, when the solid electrolyte material according to the firstembodiment is mixed with another material such as an active material, agood dispersion state of the solid electrolyte material according to thefirst embodiment and the additional material is obtained.

Method for Manufacturing Solid Electrolyte Material

The solid electrolyte material according to the first embodiment can bemanufactured by, for example, the following method.

Raw material powders are prepared and mixed so as to give a targetcomposition. The raw material powders may be, for example, halides.

As an example, when the target composition isLi_(3.0)Ti_(0.5)Mg_(0.5)F₆, LiF, TiF₄, and MgF₂ are mixed at a molarratio of about 3.0:0.5:0.5. The raw material powders may be mixed at amolar ratio adjusted in advance so as to offset a composition changethat may occur in the synthesis process.

The raw material powders are mechanochemically reacted (i.e., by amechanochemical milling method) with each other in a mixing apparatussuch as a planetary ball mill to obtain a reaction product. The reactionproduct may be heat-treated in vacuum or in an inert atmosphere.Alternatively, a mixture of the raw material powders may be heat-treatedin vacuum or in an inert atmosphere to obtain a reaction product. Theheat treatment is preferably performed, for example, at 100° C. or moreand 300° C. or less for 1 hour or more. In order to suppress thecomposition change due to heat treatment, the raw material powders arepreferably heat-treated in an airtight container such as a quartz tube.

The solid electrolyte material according to the first embodiment isobtained by these methods.

Second Embodiment

A second embodiment will now be described. The matters described in thefirst embodiment may be omitted as appropriate.

The battery according to the second embodiment includes a positiveelectrode, a negative electrode, and an electrolyte layer. Theelectrolyte layer is disposed between the positive electrode and thenegative electrode. At least one selected from the group consisting ofthe positive electrode, the electrolyte layer, and the negativeelectrode contains the solid electrolyte material according to the firstembodiment. The battery according to the second embodiment contains thesolid electrolyte material according to the first embodiment andtherefore has excellent charge and discharge characteristics. Thebattery may be an all solid battery.

FIG. 1 is a cross-sectional view of a battery 1000 according to thesecond embodiment.

The battery 1000 according to the second embodiment includes a positiveelectrode 201, an electrolyte layer 202, and a negative electrode 203.The electrolyte layer 202 is disposed between the positive electrode 201and the negative electrode 203.

The positive electrode 201 contains a positive electrode active materialparticle 204 and a solid electrolyte particle 100.

The electrolyte layer 202 contains an electrolyte material. Theelectrolyte material is, for example, a solid electrolyte material.

The negative electrode 203 contains a negative electrode active materialparticle 205 and a solid electrolyte particle 100.

The solid electrolyte particle 100 is a particle consisting of the solidelectrolyte material according to the first embodiment or a particlecontaining the solid electrolyte material according to the firstembodiment as a main component. Here, the particle containing the solidelectrolyte material according to the first embodiment as a maincomponent means a particle in which the most abundant component by massratio is the solid electrolyte material according to the firstembodiment.

The positive electrode 201 contains a material that can occlude andrelease metal ions (e.g., lithium ions). The material is, for example, apositive electrode active material (e.g., the positive electrode activematerial particle 204).

Examples of the positive electrode active material are alithium-containing transition metal oxide, a transition metal fluoride,a polyanionic material, a fluorinated polyanionic material, a transitionmetal sulfide, a transition metal oxysulfide, and a transition metaloxynitride. Examples of the lithium-containing transition metal oxideare Li(Ni,Co,Al)O₂, Li(Ni,Co,Mn)O₂, and LiCoO₂. In the presentdisclosure, the notation of “(Ni,Co,Al)” in a chemical formula shows atleast one element selected from the elements in the parentheses. Thatis, “(Ni,Co,Al)” is synonymous with “at least one selected from thegroup consisting of Ni, Co, and Al”. The same is applied to otherelements.

The positive electrode active material particle 204 may have a mediandiameter of 0.1 μm or more and 100 μm or less. When the positiveelectrode active material particle 204 has a median diameter of 0.1 μmor more, the dispersion state of the positive electrode active materialparticle 204 and the solid electrolyte particle 100 in the positiveelectrode 201 is improved. Consequently, the charge and dischargecharacteristics of the battery 1000 are improved. When the positiveelectrode active material particle 204 has a median diameter of 100 μmor less, the lithium diffusion speed in the positive electrode activematerial particle 204 is improved. Consequently, the battery 1000 canoperate at a high output.

The positive electrode active material particle 204 may have a mediandiameter larger than that of the solid electrolyte particle 100.Consequently, the dispersion state of the positive electrode activematerial particle 204 and the solid electrolyte particle 100 in thepositive electrode 201 is improved.

In order to increase the energy density and output of the battery, inthe positive electrode 201, the ratio of the volume of the positiveelectrode active material particle 204 to the sum of the volume of thepositive electrode active material particle 204 and the volume of thesolid electrolyte particle 100 may be 0.30 or more and 0.95 or less.

A coating layer may be formed on at least part of the surface of thepositive electrode active material particle 204. The coating layer maybe formed on the surface of the positive electrode active materialparticle 204, for example, before mixing a conductive assistant and abinder. Examples of the coating material included in the coating layerare a sulfide solid electrolyte, an oxide solid electrolyte, and ahalide solid electrolyte. When the solid electrolyte particle 100contains a sulfide solid electrolyte, in order to suppress the oxidativedecomposition of the sulfide solid electrolyte, the coating material maycontain the solid electrolyte material according to the firstembodiment. When the solid electrolyte particle 100 contains the solidelectrolyte material according to the first embodiment, in order tosuppress oxidative decomposition of the solid electrolyte material, thecoating material may contain an oxide solid electrolyte. As the oxidesolid electrolyte, lithium niobate, which has excellent high-potentialstability, may be used. An increase in the overpotential of the batterycan be suppressed by suppressing the oxidative decomposition of thesolid electrolyte material.

In order to increase the energy density and output of the battery, thepositive electrode 201 may have a thickness of 10 μm or more and 500 μmor less.

The electrolyte layer 202 contains an electrolyte material. Theelectrolyte material is, for example, a solid electrolyte material. Theelectrolyte layer 202 may be a solid electrolyte layer.

The electrolyte layer 202 may be constituted of the solid electrolytematerial according to the first embodiment only. Alternatively, theelectrolyte layer 202 may be constituted of only a solid electrolytematerial that is different from the solid electrolyte material accordingto the first embodiment. Examples of the solid electrolyte material thatis different from the solid electrolyte material according to the firstembodiment are Li₂MgX₄, Li₂FeX₄, Li(Al,Ga,In)X₄, Li₃(Al,Ga,In)X₆, andLiI. Here, X is at least one selected from the group consisting of F,Cl, Br, and I.

Hereinafter, the solid electrolyte material according to the firstembodiment is referred to as a first solid electrolyte material. Thesolid electrolyte material that is different from the solid electrolytematerial according to the first embodiment is referred to as a secondsolid electrolyte material.

The electrolyte layer 202 may contain not only the first solidelectrolyte material but also the second solid electrolyte material. Thefirst solid electrolyte material and the second solid electrolytematerial may be uniformly dispersed in the electrolyte layer 202. Alayer made of the first solid electrolyte material and a layer made ofthe second solid electrolyte material may be stacked along the stackingdirection of the battery 1000.

FIG. 2 is a cross-sectional view of a battery 2000 according to thesecond embodiment.

As shown in FIG. 2 , the battery 2000 may include a positive electrode201, a first electrolyte layer 212, a second electrolyte layer 222, anda negative electrode 203. That is, the electrolyte layer 202 may includea first electrolyte layer 212 and a second electrolyte layer 222. Thefirst electrolyte layer 212 is disposed between the positive electrode201 and the negative electrode 203. The second electrolyte layer 222 isdisposed between the first electrolyte layer 212 and the negativeelectrode 203.

In the battery 2000, the first electrolyte layer 212 may contain thesolid electrolyte material according to the first embodiment. Since thesolid electrolyte material according to the first embodiment has a highresistance to oxidation, the solid electrolyte material included in thesecond electrolyte layer 222 can be used without being oxidized. As aresult, the charge and discharge efficiency of a battery can beimproved.

In the battery 2000, the solid electrolyte material included in thesecond electrolyte layer 222 may have a reduction potential lower thanthat of the solid electrolyte material included in the first electrolytelayer 212. Consequently, the solid electrolyte material included in thefirst electrolyte layer 212 can be used without being reduced. As aresult, the charge and discharge efficiency of a battery can beimproved. For example, when the first electrolyte layer 212 contains thesolid electrolyte material according to the first embodiment, in orderto suppress the reductive decomposition of the solid electrolytematerial, the second electrolyte layer 222 may contain a sulfide solidelectrolyte.

In order to increase the energy density and output of the battery, theelectrolyte layer 202 may have a thickness of 1 μm or more and 1000 μmor less.

The negative electrode 203 contains a material that can occlude andrelease metal ions (e.g., lithium ions). The material is, for example, anegative electrode active material (e.g., the negative electrode activematerial particle 205).

Examples of the negative electrode active material are a metal material,a carbon material, an oxide, a nitride, a tin compound, and a siliconcompound. The metal material may be a single metal or may be an alloy.Examples of the metal material are a lithium metal and a lithium alloy.Examples of the carbon material are natural graphite, coke, carbon undergraphitization, a carbon fiber, spherical carbon, artificial graphite,and amorphous carbon. From the viewpoint of capacity density, preferableexamples of the negative electrode active material are silicon (Si), tin(Sn), a silicon compound, and a tin compound.

The negative electrode active material may be selected consideringreduction resistance of the solid electrolyte material included in thenegative electrode 203. For example, when the negative electrode 203contains the solid electrolyte material according to the firstembodiment, the negative electrode active material may be a materialthat can occlude and release lithium ions at 0.27 V or more with respectto lithium. Examples of such negative electrode active materials are atitanium oxide, an indium metal, and a lithium alloy. Examples oftitanium oxide are Li₄Ti₅O₁₂, LiTi₂O₄, and TiO₂. The solid electrolytematerial according to the first embodiment included in the negativeelectrode 203 can be suppressed from being reductively decomposed byusing the above-mentioned negative electrode active material. As aresult, the charge and discharge efficiency of a battery can beimproved.

The negative electrode active material particle 205 may have a mediandiameter of 0.1 μm or more and 100 μm or less. When the negativeelectrode active material particle 205 has a median diameter of 0.1 μmor more, the dispersion state of the negative electrode active materialparticle 205 and the solid electrolyte particle 100 in the negativeelectrode 203 is improved. Consequently, the charge and dischargecharacteristics of the battery are improved. When the negative electrodeactive material particle 205 has a median diameter of 100 μm or less,the lithium diffusion speed in the negative electrode active materialparticle 205 is improved. Consequently, the battery can operate at ahigh output.

The negative electrode active material particle 205 may have a mediandiameter larger than that of the solid electrolyte particle 100.Consequently, the dispersion state of the negative electrode activematerial particle 205 and the solid electrolyte particle 100 in thenegative electrode 203 is improved.

In order to increase the energy density and output of the battery, inthe negative electrode 203, the ratio of the volume of the negativeelectrode active material particle 205 to the sum of the volume of thenegative electrode active material particle 205 and the volume of thesolid electrolyte particle 100 may be 0.30 or more and 0.95 or less.

In order to increase the energy density and output of the battery, thenegative electrode 203 may have a thickness of 10 μm or more and 500 μmor less.

At least one selected from the group consisting of the positiveelectrode 201, the electrolyte layer 202, and the negative electrode 203may contain the second solid electrolyte material for the purpose ofenhancing the ion conductivity, chemical stability, and electrochemicalstability.

The second solid electrolyte material may be a sulfide solidelectrolyte.

Examples of the sulfide solid electrolyte are Li₂S-P₂S₅, Li₂S-SiS₂,Li₂S-B₂S₃, Li₂S-GeS₂, Li_(3.25)Ge_(0.25)P_(0.75)S₄, and Li₁₀GeP₂S₁₂.

When the electrolyte layer 202 contains the solid electrolyte materialaccording to the first embodiment, in order to suppress the reductivedecomposition of the solid electrolyte material, the negative electrode203 may contain a sulfide solid electrolyte. The solid electrolytematerial according to the first embodiment is prevented from coming intocontact with the negative electrode active material by coating thenegative electrode active material with an electrochemically stablesulfide solid electrolyte. As a result, the internal resistance of abattery can be reduced.

The second solid electrolyte material may be an oxide solid electrolyte.

Examples of the oxide solid electrolyte are:

(i) an NASICON-type solid electrolyte, such as LiTi₂(PO₄)₃ or itselement substitute;

(ii) a perovskite-type solid electrolyte, such as (LaLi)TiO₃;

(iii) an LISICON-type solid electrolyte, such as Li₁₄ZnGe₄O₁₆, Li₄SiO₄,LiGeO₄, or their element substitutes;

(iv) a garnet-type solid electrolyte, such as Li₇La₃Zr₂O₁₂ or itselement substitute; and

(v) Li₃PO₄ or its N-substitute.

As described above, the second solid electrolyte material may be ahalide solid electrolyte.

Examples of the halide solid electrolyte are Li₂MgX₄, Li₂FeX₄,Li(Al,Ga,In)X₄, Li₃(Al,Ga,In)X₆, and LiI. Here, X is at least oneselected from the group consisting of F, Cl, Br, and I.

Other examples of the halide solid electrolyte material are compoundsrepresented by Li_(a)Me_(b)Y_(c)X₆. Here, a+mb+3c=6 and c>0 aresatisfied. Me is at least one selected from the group consisting ofmetal elements excluding Li and Y and metalloid elements, and mrepresents the valence of Me. The “metalloid elements” are B, Si, Ge,As, Sb, and Te. The “metal elements” are all elements included in Groups1 to 12 of the periodic table (however, hydrogen is excluded) and allelements included in Groups 13 to 16 in the periodic table (however, B,Si, Ge, As, Sb, Te, C, N, P, O, S, and Se are excluded).

In order to enhance the ion conductivity of the halide solid electrolytematerial, Me may be at least one selected from the group consisting ofMg, Ca, Sr, Ba, Zn, Sc, Al, Ga, Bi, Zr, Hf, Ti, Sn, Ta, and Nb. Thehalide solid electrolyte may be Li₃YCl₆ or Li₃YBr₆.

The second solid electrolyte material may be an organic polymer solidelectrolyte.

Examples of the organic polymer solid electrolyte are compounds ofpolymer compounds and lithium salts.

The polymer compound may have an ethylene oxide structure. A polymercompound having an ethylene oxide structure can contain a large amountof a lithium salt and can therefore further enhance the ionconductivity.

Examples of the lithium salt are LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiSO₃CF₃,LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiN(SO₂CF₃)(SO₂C₄F₉), and LiC(SO₂CF₃)₃. Onelithium salt selected from these salts may be used alone. Alternatively,a mixture of two or more lithium salts selected from these salts may beused.

At least one selected from the group consisting of the positiveelectrode 201, the electrolyte layer 202, and the negative electrode 203may contain a nonaqueous electrolyte liquid, a gel electrolyte, or anionic liquid for the purpose of facilitating the transfer of lithiumions and improving the output characteristics of the battery.

The nonaqueous electrolyte liquid includes a nonaqueous solvent and alithium salt dissolved in the nonaqueous solvent.

Examples of the nonaqueous solvent are a cyclic carbonate ester solvent,a chain carbonate ester solvent, a cyclic ether solvent, a chain ethersolvent, a cyclic ester solvent, a chain ester solvent, and a fluorinesolvent. Examples of the cyclic carbonate ester solvent are ethylenecarbonate, propylene carbonate, and butylene carbonate. Examples of thechain carbonate ester solvent are dimethyl carbonate, ethyl methylcarbonate, and diethyl carbonate. Examples of the cyclic ether solventare tetrahydrofuran, 1,4-dioxane, and 1,3-dioxolane. Examples of thechain ether solvent are 1,2-dimethoxyethane and 1,2-diethoxyethane. Anexample of the cyclic ester solvent is γ-butyrolactone. An example ofthe chain ester solvent is ethyl acetate. Examples of the fluorinesolvent are fluoroethylene carbonate, methyl fluoropropionate,fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethylenecarbonate. One nonaqueous solvent selected from these solvents may beused alone. Alternatively, a mixture of two or more nonaqueous solventsselected from these solvents may be used.

Examples of the lithium salt are LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiSO₃CF₃,LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiN(SO₂CF₃)(SO₂C₄F₉), and LiC(SO₂CF₃)₃. Onelithium salt selected from these salts may be used alone. Alternatively,a mixture of two or more lithium salts selected from these salts may beused. The concentration of the lithium salt is, for example, within arange of 0.5 mol/L or more and 2 mol/L or less.

As the gel electrolyte, a polymer material impregnated with a nonaqueouselectrolyte liquid can be used. Examples of the polymer material arepolyethylene oxide, polyacrylonitrile, polyvinylidene fluoride,polymethyl methacrylate, and a polymer having an ethylene oxide bond.

Examples of the cation included in the ionic liquid are:

(i) aliphatic chain quaternary salts, such as tetraalkylammonium andtetraalkylphosphonium;

(ii) aliphatic cyclic ammoniums, such as pyrrolidiniums, morpholiniums,imidazoliniums, tetrahydropyrimidiniums, piperaziniums, andpiperidiniums; and

(iii) nitrogen-containing heterocyclic aromatic cations, such aspyridiniums and imidazoliums.

Examples of the anion included in the ionic liquid are PF₆ ⁻, BF₄ ⁻,SbF₆ ⁻, AsF₆ ⁻, SO₃CF₃ ⁻, N(SO₂CF₃)₂ ⁻, N(SO₂C₂F₅)₂ ⁻,N(SO₂CF₃)(SO₂C₄F₉)⁻, and C(SO₂CF₃)₃ ⁻.

The ionic liquid may contain a lithium salt.

At least one selected from the group consisting of the positiveelectrode 201, the electrolyte layer 202, and the negative electrode 203may contain a binder for the purpose of improving the adhesion betweenindividual particles.

Examples of the binder are polyvinylidene fluoride,polytetrafluoroethylene, polyethylene, polypropylene, an aramid resin,polyamide, polyimide, polyamideimide, polyacrylonitrile, polyacrylicacid, polyacrylic acid methyl ester, polyacrylic acid ethyl ester,polyacrylic acid hexyl ester, polymethacrylic acid, polymethacrylic acidmethyl ester, polymethacrylic acid ethyl ester, polymethacrylic acidhexyl ester, polyvinyl acetate, polyvinylpyrrolidone, polyether,polyether sulfone, hexafluoropolypropylene, styrene butadiene rubber,and carboxymethyl cellulose. As the binder, a copolymer can also beused. Examples of such the binder are copolymers of two or morematerials selected from the group consisting of tetrafluoroethylene,hexafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether,vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene,pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid, andhexadiene. A mixture of two or more materials selected from theabove-mentioned materials may be used as the binder.

At least one of the positive electrode 201 and the negative electrode203 may contain a conductive assistant for reducing the electronresistance.

Examples of the conductive assistant are:

(i) graphites, such as natural graphite and artificial graphite;

(ii) carbon blacks, such as acetylene black and Ketjen black;

(iii) conductive fibers, such as carbon fibers and metal fibers;

(iv) carbon fluoride;

(v) metal powders, such as aluminum;

(vi) conductive whiskers, such as zinc oxide and potassium titanate;

(vii) conductive metal oxides, such as titanium oxide; and

(viii) conductive polymer compounds, such as polyanion, polypyrrole, andpolythiophene. In order to reduce the cost, the conductive assistant ofthe above (i) or (ii) may be used.

Examples of the shape of the battery according to the second embodimentare coin type, cylindrical type, square type, sheet type, button type,flat type, and stacked type.

The battery according to the second embodiment may be manufactured by,for example, preparing a material for forming a positive electrode, amaterial for forming an electrolyte layer, and a material for forming anegative electrode and producing a stack of a positive electrode, anelectrolyte layer, and a negative electrode disposed in this order by aknown method.

EXAMPLES

The present disclosure will now be described in more detail withreference to Examples and Comparative Examples.

Example 1 Production of Solid Electrolyte Material

LiF, TiF₄, and MgF₂ were prepared as raw material powders at a molarratio, LiF:TiF₄:MgF₂, of 3.0:0.5:0.5 in an argon atmosphere having a dewpoint of −60° C. or less (hereinafter, referred to as “dry argonatmosphere”). These materials were pulverized and mixed in a mortar. Theresulting mixture was subjected to milling treatment with a planetaryball mill at 500 rpm for 12 hours. Thus, a solid electrolyte materialpowder of Example 1 was obtained. The solid electrolyte material ofExample 1 had a composition represented by Li_(3.0)Ti_(0.5)Mg_(0.5)F₆.

Evaluation of Ion Conductivity

FIG. 3 is a schematic view of a compression molding dies 300 used forevaluation of the ion conductivity of a solid electrolyte material.

The compression molding dies 300 included a punch upper part 301, a die302, and a punch lower part 303. The die 302 was made of insulatingpolycarbonate. The punch upper part 301 and the punch lower part 303were made of electron-conductive stainless steel.

The ion conductivity of the solid electrolyte material of Example 1 wasevaluated using the compression molding dies 300 shown in FIG. 3 by thefollowing method.

The powder of the solid electrolyte material of Example 1 was loadedinside the compression molding dies 300 in a dry atmosphere having a dewpoint of −30° C. or less. A pressure of 400 MPa was applied to the solidelectrolyte material of Example 1 inside the compression molding dies300 using the punch upper part 301 and the punch lower part 303.

The punch upper part 301 and the punch lower part 303 were connected toa potentiostat (Princeton Applied Research, VersaSTAT4) equipped with afrequency response analyzer while applying the pressure. The punch upperpart 301 was connected to the working electrode and the potentialmeasurement terminal. The punch lower part 303 was connected to thecounter electrode and the reference electrode. The impedance of a solidelectrolyte material was measured by an electrochemical impedancemeasurement method at room temperature.

FIG. 4 is a graph showing Cole-Cole plots obtained by impedancemeasurement of the solid electrolyte material of Example 1.

In FIG. 4 , the real value of impedance at the measurement point wherethe absolute value of the phase of the complex impedance was thesmallest was regarded as the resistance value to ion conduction of thesolid electrolyte material. The ion conductivity was calculated usingthe resistance value based on a mathematical expression (2):σ=(R_(SE)×S/t)⁻¹, where σ represents ion conductivity; S represents thecontact area of a solid electrolyte material with the punch upper part301 (equal to the cross-sectional area of the hollow part of the die 302in FIG. 3 ); R_(SE) represents the resistance value of the solidelectrolyte material in impedance measurement; and t represents thethickness of the solid electrolyte material (i.e., in FIG. 3 , thethickness of the layer formed from the powder 101 of the solidelectrolyte material).

The ion conductivity of the solid electrolyte material of Example 1measured at 25° C. was 2.33×10⁻⁶ S/cm.

Production of Battery

The solid electrolyte material of Example 1 and LiCoO₂ as an activematerial were prepared at a volume ratio of 30:70 in a dry argonatmosphere. These materials were mixed in an agate mortar. Thus, apositive electrode mixture was obtained.

Subsequently, LiCl and YCl₃ were prepared at a molar ratio, LiCl:YCl₃,of 3:1. These materials were pulverized and mixed in a mortar. Theresulting mixture was subjected to milling treatment with a planetaryball mill at 500 rpm for 12 hours. Thus, a halide solid electrolytehaving a composition represented by Li₃YCl₆ (hereinafter, referred to as“LYC”) was obtained.

LYC (60 mg), the solid electrolyte material (26 mg) of Example 1, andthe above-described positive electrode mixture (9.1 mg) were stacked inthis order in an insulating tube having an inner diameter of 9.5 mm. Apressure of 300 MPa was applied to the resulting stack to form a secondelectrolyte layer, a first electrolyte layer, and a positive electrode.That is, a first electrolyte layer made of the solid electrolytematerial of Example 1 was sandwiched between a second electrolyte layerand a positive electrode. The second electrolyte layer and the firstelectrolyte layer had thicknesses of 450 μm and 150 μm, respectively.

Subsequently, metal In (thickness: 200 μm) was stacked on the secondelectrolyte layer. A pressure of 80 MPa was applied to the resultingstack to form a negative electrode.

Subsequently, a current collector made of stainless steel was attachedto the positive electrode and the negative electrode, and a currentcollecting lead was attached to the current collector.

Finally, the inside of the insulating tube was isolated from the outsideatmosphere using an insulating ferrule to seal the inside of the tube.Thus, a battery of Example 1 was obtained.

Charge and Discharge Test

FIG. 5 is a graph showing the initial discharge characteristics of thebattery of Example 1. The initial charge and discharge characteristicswere measured by the following method.

The battery of Example 1 was placed in a thermostat of 85° C.

The battery of Example 1 was charged at a current density of 27 μA/cm²until the voltage reached 3.6 V. The current density corresponded to0.02 C rate.

Subsequently, the battery of Example 1 was discharged at a currentdensity of 27 μA/cm² until the voltage reached 1.9 V.

As the results of the charge and discharge test, the battery of Example1 had an initial discharge capacity of 900.76 μAh.

Examples 2 to 16 Production of Solid Electrolyte Material

In Examples 2 to 15, LiF, TiF₄, and MgF₂ were prepared as raw materialpowders at a molar ratio, LiF:TiF₄: MgF₂, of {6−(4−2x)b}:(1−x)b:xb.

In Example 16, LiF, TiF₄, and CaF₂ were prepared as raw material powdersat a molar ratio, LiF:TiF₄:CaF₂, of {6−(4−2x)b}:(1−x)b:xb.

Solid electrolyte materials of Examples 2 to 16 were obtained as inExample 1 except the above matters.

Regarding the solid electrolyte materials of Examples 2 to 16, thevalues of x, b, and Li/(Ti+M) molar ratio are shown in Table 1.

Evaluation of Ion Conductivity

The ion conductivities of the solid electrolyte materials of Examples 2to 16 were measured as in Example 1. The measurement results are shownin Table 1.

Charge and Discharge Test

Batteries of Examples 2 to 16 were obtained as in Example 1 using thesolid electrolyte materials of Examples 2 to 16.

The batteries of Examples 2 to 16 were subjected to a charge anddischarge test as in Example 1. The batteries of Examples 2 to 16 werewell charged and discharged as in the battery of Example 1.

Comparative Example 1

As the solid electrolyte material, LiBF₄ was used instead ofLi_(3.0)Ti_(0.5)Mg_(0.5)F₆.

The ion conductivity of LiBF₄ was measured as in Example 1. The ionconductivity measured at 25° C. was 6.67×10⁻⁹ S/cm.

A battery of Comparative Example 1 was obtained as in Example 1 usingLiBF₄ as the solid electrolyte material.

The battery of Comparative Example 1 was subjected to a charge anddischarge test as in Example 1. As a result, the battery of ComparativeExample 1 had an initial discharge capacity of 0.01 μAh or less. Thatis, the battery of Comparative Example 1 was neither charged nordischarged.

Table 1 shows the solid electrolyte materials of Examples 1 to 16 andComparative Example 1 and each evaluation result.

TABLE 1 Ion Li/(Ti + M) conductivity Composition formula x b M molarratio [S/cm] Example 1 Li_(3.0)Ti_(0.5)Mg_(0.5)F₆ 0.5 1 Mg 3.00 2.33 ×10 ⁻⁶ Example 2 Li_(3.43)Ti_(0.43)Mg_(0.43)F₆ 0.5 0.86 Mg 3.99 7.88 ×10⁻ ⁷ Example 3 Li_(2.4)Ti_(0.6)Mg_(0.6)F₆ 0.5 1.2 Mg 2.00 9.01 × 10⁻ ⁷Example 4 Li_(1.5)Ti_(0.75)Mg_(0.75)F₆ 0.5 1.5 Mg 1.00 6.35 × 10⁻ ⁸Example 5 Li_(0.86)Ti_(0.86)Mg_(0.86)F₆ 0.5 1.71 Mg 0.50 4.10 × 10⁻ ⁸Example 6 Li_(3.2)Ti_(0.4)Mg_(0.6)F₆ 0.6 1 Mg 3.20 2.46 × 10⁻ ⁷ Example7 Li_(2.8)Ti_(0.6)Mg_(0.4)F₆ 0.4 1 Mg 2.80 4.67 × 10⁻ ⁷ Example 8Li_(2.4)Ti_(0.8)Mg_(0.2)F₆ 0.2 1 Mg 2.40 3.17 × 10⁻ ⁷ Example 9Li_(2.2)Ti_(0.9)Mg_(0.1)F₆ 0.1 1 Mg 2.20 2.99 × 10⁻ ⁷ Example 10Li_(3.6)Ti_(0.2)Mg_(0.8)F₆ 0.8 1 Mg 3.60 3.14 × 10⁻ ⁷ Example 11Li_(3.3)Ti_(0.45)Mg_(0.45)F₆ 0.5 0.9 Mg 3.67 4.97 × 10⁻ ⁷ Example 12Li_(3.6)Ti_(0.4)Mg_(0.4)F₆ 0.5 0.8 Mg 4.50 7.26 × 10⁻ ⁷ Example 13Li_(2.7)Ti_(0.55)Mg_(0.55)F₆ 0.5 1.1 Mg 2.45 4.53 × 10⁻ ⁷ Example 14Li_(2.1)Ti_(0.65)Mg_(0.65)F₆ 0.5 1.3 Mg 1.62 4.09 × 10⁻ ⁷ Example 15Li_(2.1)Ti_(0.95)Mg_(0.05)F₆ 0.05 1 Mg 2.10 2.95 × 10⁻ ⁷ Example 16Li_(3.0)Ti_(0.5)Ca_(0.5)F₆ 0.5 1 Ca 3.00 1.91 × 10⁻ ⁸ Comparative LiBF₄— — — 1.00 6.67 × 10⁻ ⁹ Example 1

Consideration

The solid electrolyte materials of Examples 1 to 16 have high ionconductivities of 1×10⁻⁸ S/cm or more at room temperature. In contrast,the solid electrolyte material of Comparative Example has a low ionconductivity of less than 1×10⁻⁸ S/cm.

As obvious by comparing Examples 1 to 3 and 11 to 14 with Examples 4 and5, when 0.8≤b≤1.3 is satisfied, the ion conductivity of a solidelectrolyte material is further enhanced.

As obvious by comparing Example 1 with Example 16, when M is Mg ratherthan Ca, the ion conductivity of a solid electrolyte material is higher.

All the batteries of Examples 1 to 16 were charged and discharged at 85°C. In contrast, the battery of Comparative Example 1 was neither chargednor discharged.

Since the solid electrolyte materials of Examples 1 to 16 do not containsulfur, hydrogen sulfide is not generated.

As described above, the solid electrolyte material of the presentdisclosure has a high lithium ion conductivity and is suitable forproviding a battery that can be well charged and discharged.

The solid electrolyte material of the present disclosure is used in, forexample, an all solid lithium ion secondary battery.

What is claimed is:
 1. A solid electrolyte material consistingessentially of Li, Ti, M, and F, wherein M is at least one selected fromthe group consisting of Mg and Ca.
 2. A solid electrolyte materialcomprising Li, Ti, M, and F, wherein M is at least one selected from thegroup consisting of Mg and Ca, and a ratio of the amount of Li to thesum of the amounts of Ti and M is 0.5 or more and 4.5 or less.
 3. Thesolid electrolyte material according to claim 1, wherein the solidelectrolyte material is represented by a composition formula (1):Li_(6-(4-2x)b)(Ti_(1-x)M_(x))_(b)F₆, wherein 0<x<1 and 0<b≤3 aresatisfied.
 4. The solid electrolyte material according to claim 3,wherein a mathematical expression: 0.05≤x≤0.8 is satisfied.
 5. The solidelectrolyte material according to claim 3, wherein M is Mg, and amathematical expression: 0.05≤x≤0.6 is satisfied.
 6. The solidelectrolyte material according to claim 3, wherein M is Ca, and amathematical expression: x=0.5 is satisfied.
 7. The solid electrolytematerial according to claim 3, wherein a mathematical expression:0.80≤b≤1.71 is satisfied.
 8. A battery comprising: a positive electrode;a negative electrode; and an electrolyte layer disposed between thepositive electrode and the negative electrode, wherein at least oneselected from the group consisting of the positive electrode, thenegative electrode, and the electrolyte layer contains the solidelectrolyte material according to claim
 1. 9. The battery according toclaim 8, wherein the electrolyte layer includes a first electrolytelayer and a second electrolyte layer, the first electrolyte layer isdisposed between the positive electrode and the negative electrode, thesecond electrolyte layer is disposed between the first electrolyte layerand the negative electrode, and the first electrolyte layer contains thesolid electrolyte material.